For the operation of a crane, in particular with regard to safety, it is extraordinarily important to know the overall length of a single-part jib or also the overall length of a telescopic jib. The load at which the jib can be operated is dependent upon both the overall length and also the jib angle and optionally further parameters.
For determination of the length of the jib, in particular in the case of telescopic jibs with a plurality of jib elements, it is already known to determine the current overall length of the jib by the length of a cable that is entrained when the jib is retracted or extended. This cable is wound or unwound on a drum, wherein the number of turns is a measure of the wound or unwound length of cable and thus indirectly is also a measure of the overall length of the jib. Although such cable-guided systems have proved successful in practice, they have the disadvantage that during the service life of the crane they are subject to wear and moreover they involve additional weight. If it is to be possible to determine the jib length with sufficient precision, especially in the case of greater jib lengths, it is necessary to implement additional measures (for example in order to prevent sagging of the cable).
A wear-free method of determination of the overall length of a jib of a crane is already known from DE 10 2006 025 002 [U.S. Pat. No. 8,181,798]. In this connection the jib length is determined by wireless technology, in particular by RFID technology. This system does indeed overcome the disadvantages of the cable-guided system, specifically the precision is improved, the weight is reduced and also wear is eliminated. However, the wireless-based system has the disadvantage that it is not interference-free to safety-critical standards without further additional and thus costly provisions, since extraneous interference signals (for example by mobile wireless, radiotelephony and the like) can interfere with the signal transmission. Thus for the safety-related operation of a crane it is not ensured that the jib length can be determined reliably by such a system.